The present disclosure relates to an image processing apparatus, an imaging apparatus, an image processing method, and a program. More particularly, the present disclosure relates to an image processing apparatus, an imaging apparatus, an image processing method, and a program for performing image correction such as reducing noise included in an image.
An image sensor, such as a CCD sensor or a CMOS (complementary metal oxide semiconductor), that is used in a video camera or a still camera accumulates charge based on the amount of incident light, and performs photoelectric conversion for outputting an electric signal corresponding to the accumulated charge. A color filter for individually accumulating signals corresponding to specific wavelength light, namely, a specific color, for each pixel unit is mounted in such an image sensor. For example, a filter having a Bayer array configured from the respective colors of red, green, and blue (RGB) is often used.
On the other hand, recently, pixels are becoming smaller due to the decreasing size of image sensors and the increasing number of pixels. An image sensor having such a high density of pixels is important when capturing high-resolution images.
However, the amount of charge that can be accumulated per pixel in an image sensor having such a high density of pixels is reduced, so that there is the problem that the noise amount included in the signal from each pixel is comparatively greater.
For example, especially for images captured under low illuminance, the S/N ratio of the color signals (chroma signals) deteriorates due to the carrying out of a large gain adjustment as well as gamma correction and the like, which leads to substantial unevenness in the color signals, namely, chroma noise, of the output image.
As a related-art technology disclosing reduction processing of such chroma noise, for example, JP 2006-302023-A discloses a configuration that executes correction processing based on the pixel values of surrounding pixels using a bilateral filter as a smoothing filter to reduce the noise included in the pixels.
However, since a reference pixel area of a bilateral filter is a comparatively narrow pixel area, it is difficult in correction processing that uses a bilateral filter to remove ultra-low frequency chroma noise spread out over a wide area.
Further, JP 2010-239271-A discloses a configuration that performs noise reduction by executing interpolation processing, in which the pixel values in a noise area are set by utilizing the pixel values of representative pixels obtained by calculating a statistic consisting of two elements, “color” and “position”, from an image and determining the representative pixels in area units based on analysis of the calculated statistic.
With this configuration, it is possible to remove the low-frequency chroma noise that is not resolved by the above-described bilateral filter.
However, in this configuration, there is the problem that if noise is included in the selected representative pixels themselves, the noise reduction effect substantially deteriorates.